[media] DocBook: improve STREAMON/OFF documentation

Specify that STREAMON/OFF should return 0 if the stream is already
started/stopped.
The spec never specified what the correct behavior is. This ambiguity
was resolved during the 2012 Media Workshop.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>